A usable, macroscopic model was developed to describe apparent protein adsorption equilibrium at hydrophobic solid-water interfaces. In particular, the adsorbed mass of protein was expressed in terms of the following macroscopic properties : the partial molar area occupied by protein at the interface; the partial molar volume of protein in solution; the work of adhesion per unit area; the minimum surface area required by an adsorbing protein; and the apparent equilibrium concentration of protein in solution. Adsorption at hydrophobic interfaces was considered to occur in the absence of specific electrostatic and biochemical interactions, according to a pseudo-equilibrium between protein in solution and that adsorbed in some altered state. The work of adhesion between protein and surface was calculated assuming that the equilibrium spreading pressure of protein could be used in estimation of protein interfacial energy. The Gibbs free energy of unfolding was used in the estimation of protein flexibility, needed for the calculation of equilibrium spreading pressure with the selected equation of state. Good agreement was observed between the model and experimentally measured isotherms for the milk proteins alpha-lactalbumin, beta-lactoglobulin, and bovine serum albumin at hydrophobic silica.